This invention relates to a method and apparatus for monitoring selected electrical characteristics of a battery used to power an electrical system.
Batteries are used in many critical applications. For example, stationary storage batteries may be used to power equipment and components in electrical power generating and/or distribution substations. When used, the batteries must supply the needed current and power to operate circuits used to enable the orderly shut down or de-energization, (or even the re-energization and turn-on) of critical control circuits within the substation. It is therefore important that the battery be in a condition to supply the power needed to operate the system reliably. However, with the battery connected in circuit, it is difficult to determine the condition of the battery and/or whether it is at, or close to, a failure point. In the case of unmanned sites, such as a remote substation, skilled personnel may be routinely dispatched to the remote locations to check out the condition of these batteries. However, this is expensive and inefficient.
These batteries are xe2x80x9cheavy dutyxe2x80x9d batteries and may be, for example, of the lead acid type, nickel cadmium type and/or any other suitable type. As a storage battery ages as a function of usage, care and temperature, electro-chemical changes take place within the battery and also at the connections to the battery. The electro-chemical changes may give rise to an increase in the resistance of the battery that may inhibit the battery from delivering the necessary current. Alternatively, the electro-chemical changes may give rise to an unwanted decrease (e.g., a short) in the resistance of the battery. It is therefore desirable and/or necessary to continuously monitor the resistance of the battery and/or the ability of the battery to provide the required power output to a load.
Some known techniques to measure battery resistance rely on the injection of AC currents into the battery for measuring both its absolute impedance and changes in the impedance of the battery. However, the circuitry needed to accurately measure the change in the impedance to a required degree of resolution renders these techniques prohibitively expensive in most applications.
These problems are overcome in circuits and systems embodying the invention.
In circuits and systems embodying the invention the condition of a battery, supplying a load current to a load, may be tested and monitored while the battery is connected to the load and without interrupting that operation. A selectively enabled switch causes a test current to selectively flow through the battery and a test resistor of known value. The voltage across the test resistor is sensed to determine the value of the test current. The voltage across the battery is measured under two conditions, with the test current and without the test current flowing through it. Making and processing these measurements and determining the value of the test current enable the internal resistance of the battery to be calculated at any point in time and to be monitored over time.